Question

I need Help to Write a function in C that will Decrypt at least one word with a substitution cipher given cipher text and key

My Current Code is:

void SubDecrypt(char *message, char *encryptKey) {
int iteration;
int iteration_Num_Two = 0;
int letter;
printf("Enter Encryption Key: \n");                                                           //Display the message to enter encryption key
scanf("%s", encryptKey);                                                                   //Input the Encryption key
for (iteration = 0; message[iteration] != '0'; iteration++)                               //loop will continue till message reaches to end
{
letter = message[iteration];                                                       //Assign the first character of message to letter
if (letter >= 'A' && letter <= 'Z')                                           //Check the letter is between 'A' to 'Z' i.e/ upper case character or not
{
letter = letter - 32;                                               //Move the letter to 32 characters back
}
if (letter >= 65 && letter <= 90) {                                   //After subtraction if the letter lies in the range 65 to 90(inclusive)
  
for (iteration_Num_Two = 0; iteration_Num_Two < 27; iteration_Num_Two++) //loop will continue from 0 to 26
{
if (message[iteration] == encryptKey[iteration_Num_Two])       //Check if the message[iteration] and encryptKey[iteration_Num_Two] will equal then terminate the loop
{
break;
}
}
message[iteration] = 'A' + iteration_Num_Two;                           //Set the message[iteration] to the value of iteration_Num_Two+'A'(uppercase) i.e/ 65+iteration_Num_Two
  
}
printf("%s\n", message);                                                           //Display the message

}
;
printf("CipherText message: %s\n", message);                                                   //Display the cipher text
printf("%s\n", encryptKey);                                                                   //Display the encryptKey
}

The Output is:

Please Enter the The Text You Wish to Encrypt/Decrypt: SCGGW Select option to be completed 1. Encrypt the entered mesaage with a rotation cipher with a given key 2. Decryption of a measage encrypted with a rotation cipher given cipher text and rotation amount 3. Encryption of a message with a aubatitution cipher given measage text and alphabet aubatitution . Decryption of a meaaage encrypted with a subatitution cipher given cipher text and aubatitutions 5. Decryption of a measage encrypted with a rotation cipher given cipher text only Have Chosen option4 nter Encryption Key: SCGGW SCGGW SCGGW SCGGW SCGGW SCGGW SCGGW SCGGW SCGGW ipherText mcssage: SCGGW KIMOCDHSOPAGUJWWYERPLTZBXNV

PLEASE BE ADVISED IVE ONLY PROVIDED MY CODE FOR THE FUNCTION NOT MY MAIN USER CODE. COULD YOU PLEASE SHOW ME WHERE IVE GONE WRONG

Answer 1

You have made two or three mistakes , all these mistakes have been corrected in the below code

#include<stdio.h>

void SubDecrypt(char *message, char *encryptKey) {
int iteration;
int iteration_Num_Two = 0;
int letter;
printf("Enter Encryption Key: \n"); //Display the message to enter encryption key
scanf("%s", encryptKey); //Input the Encryption key
for (iteration = 0; message[iteration] != '\0'; iteration++) //loop will continue till message reaches to end
{
letter = message[iteration]; //Assign the first character of message to letter
if (letter >= 'a' && letter <= 'z') //Check the letter is between 'a' to 'z' i.e/ lower case character or not
{
letter = letter - 32; //Move the letter to 32 characters back for converting into upper case letter
}
if (letter >= 65 && letter <= 90) { //After subtraction if the letter lies in the range 65 to 90(inclusive)
  
for (iteration_Num_Two = 0; iteration_Num_Two < 27; iteration_Num_Two++) //loop will continue from 0 to 26
{
if (letter == encryptKey[iteration_Num_Two]) //Check if the letter and encryptKey[iteration_Num_Two] will equal then terminate the loop
{
break;
}
}
if(message[iteration]>='a' && message[iteration]<='z') //WHEN our original message is small so convert to equivalent small letter
message[iteration]='a'+iteration_Num_Two;
else
message[iteration] = 'A' + iteration_Num_Two; //Set the message[iteration] to the value of iteration_Num_Two+'A'(uppercase) i.e/ 65+iteration_Num_Two
  
}
printf("%s\n", message); //Display the message

}
;
printf("CipherText message: %s\n", message); //Display the cipher text
printf("%s\n", encryptKey); //Display the encryptKey
}

int main()
{
   char message[]="SCGGW";
   char encryptKey[100];
   SubDecrypt(message,encryptKey);
}

you can compare with your code to find your mistakes , also the key is in upper case that is why i've added the code where you need to first convert lower case to upper case ( you were doing for uppper case) .

Also in your output you have entered a wrong key which contain two consecutive W so remove that one W .

the correct output is shown below

PS D: \mingw-w64 1686-8.1.0-posix-dwarf-rt_v6-revo\mingw32\bin> ./a.exe Enter Encryption Key: KIMQCDHSOFAGUJWYERPLTZBXNV HCGGW HEGGW HELGW HELLW HELLO cipherText message: HELLO KIMQCDHSOFAGUJWYERPLTZBXNV PS D: \mingw-w64 1686-8.1.0-posix-dwarf-rt_v6-revo\ mingw32\bin> .

I hope this will help you so please give positive ratings :))